Mechanical and optical properties in precipitated regions of alumina‐rich magnesium aluminate spinel

Abstract The toughening and strengthening of transparent ceramics is challenging because microstructural alterations typically lead to light scattering. Here, controlled precipitation of α‐Al ₂ O ₃ from nonstoichiometric spinel is explored to demonstrate unique control over the evolution of second phase Al ₂ O ₃ and how the microstructure might be altered to enhance fracture toughness while minimizing light scatter. Alumina‐rich magnesium aluminate spinel, MgO·nAl ₂ O ₃ , where n=2, was hot pressed and HIP ed to produce fully dense, single‐phase material. The material was then heat treated in air at 1573 K for up to 20 hours to create a two‐phase spinel‐Al ₂ O ₃ composite. The fracture toughness varies from 0.88 to 2.47 MP a√m depending on the microstructure; enhanced toughness at the surface was due to increased crack tortuosity at phase boundaries, but residual tensile stresses were observed in the interior of the material. Precipitation causes local volume contraction and the formation of porosity, decreasing optical transmission, especially for heat treatment times longer than 5 hours.